Shivi is 20% less efficient than Manik.

Manik takes 60 DAYS to complete a work.

∴ Number of days taken by Shivi to finish a work = $(\FRAC{{60}}{{100 - 20}} \times 100 = 75\;days)$

By Unitary method, Work done by Shivi in ONE day = 1/75

Similarly, work done by Manik in one day = 1/60

Work done by Shivi on first two days = 2/75 = 2/75

Work done by Manik on third day =$(\;\frac{1}{{60}})$

∴ TOTAL work done in a span of three days = $(\frac{2}{{75}} + \frac{1}{{60}} = \frac{{8 + 5}}{{300}} = \frac{13}{{300}})$

∴ Total number of complete spans = Integer part of $(\frac{{300}}{13} = 23)$

Remaining work = $(1 - \left( {23 \times \frac{13}{{300}}} \right) = \frac{1}{{300}})$

After completion of three day cycle, once again Shivi will work on first two days.

∴ Time taken by Shivi to complete $(\frac{1}{300})$work = $(\frac{1}{{300}} \times 75 = 1/4\;days)$ which is less than 2 days.

∴ Approximate total number of days taken to complete the work = (23 × 3 + (1/4))

= 69.25 days ≈ 70 days